1. Field of the Invention
This invention relates generally to heaters. More specifically, the present invention relates to a linear heater which can be advantageously used in an office automation apparatus such as a photocopier or electrophotographic printer for fixing images on a paper sheet for example.
2. Description of the Prior Art
Various types of linear heaters are known for fixing images (deposited toner) on a paper sheet in photocopiers or electrophotographic printers (e.g. laser beam printer). Typical examples include a lamp heater and a roller heater.
However, the lamp heater and roller heater are equally disadvantageous in that there is a limitation in reducing size (thickness) and cost. Further, the lamp heater is easily damaged due to the nature of material, whereas the roller heater has a complicated structure due to the necessity of incorporating plural heating elements within the roller.
To eliminate the problems of the conventional heaters, it has been proposed to use a linear heater for fixing images on a paper sheet in electrophotography, as disclosed for example in U.S. Pat. No. 5,068,517. For the convenience of explanation, a typical arrangement of a prior art linear heater is shown in FIGS. 7 and 8 of the accompanying drawings.
As shown in FIGS. 7 and 8, the typical prior art linear heater comprises an elongate ceramic insulating substrate A1 formed with a printed resistor strip A2 extending longitudinally of the substrate. Each end of the resistor strip A2 is provided with a conductor terminal pads A3 made of e.g. silver for connection to a power source (not shown). The resistor strip A2 is covered by a heat-resistant protective coating A4 for providing electrical insulation in addition to insuring smooth contact with a sheet material to be heated. The resistor strip A2, which is made of silver-palladium alloy for example, generates heat when a current is passed therethrough.
Obviously, the prior art linear heater is very simple in arrangement. Further, the linear heater can be made very thin and light by reducing the thickness of the substrate A1. Moreover, the linear heater is also advantageous in that the time required for warming up is very short. However, the prior art linear heater still has the following problem.
The protective coating A4 is formed by depositing a paste material over the resistor strip A2 and thereafter allowing the paste to harden by drying. Due to the viscous nature of the paste, it tends to flow down toward the substrate A1 along the respective longitudinal margins A2' of the resistor strip A2 before complete hardening, as shown in FIG. 8. As a result, the hardened protective coating A4 will have a larger thickness T1 immediately above the resistor strip A2 but a smaller thickness T2 at positions corresponding to the longitudinal margins A2' of the resistor strip A2.
Obviously, if the thickness T2 of the protective coating A4 at the longitudinal margins A2' of the resistor strip A2 is too small, the protective coating A4 fails to provide intended insulation at these positions. Thus, to be safer, the protective coating A4 as a whole must be rendered thick enough. However, such a solution gives rise to another problem that the increased thickness T1 of the protective coating A4 immediately above the resistor strip A2 hinders thermal transmission from the resistor strip A2, consequently failing to provide an intended heating function.